JP7175973B2 - Separation membrane stability evaluation method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for evaluating the stability of a separation membrane, and more particularly to a method for evaluating the stability of a separation membrane that can predict in advance the explosion safety of the separation membrane prior to the nail penetration test of the battery.

This application claims priority based on Korean Patent Application No. 10-2018-0035897 filed on March 28, 2018, and all contents disclosed in the specification and drawings of that application are incorporated into this application. .

Recently, there has been an increasing interest in energy storage technology. Efforts in the research and development of electrochemical devices are gradually taking shape as the fields of application expand to the energy of mobile phones, camcorders, notebook PCs, and even electric vehicles. Electrochemical devices are a field that has received the most attention from this point of view, and among them, the development of chargeable/dischargeable secondary batteries is a focus of interest. Recently, in the development of such batteries, in order to improve the capacity density and specific energy, research and development on new electrode and battery designs are proceeding.

Among the secondary batteries currently in use, lithium secondary batteries developed in the early 1990s are conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries that use aqueous electrolytes. Compared to , the operating voltage is high and the energy density is much higher. However, the lithium ion battery has drawbacks such as safety problems such as ignition and explosion due to the use of an organic electrolyte, and complicated manufacturing.

Recent lithium-ion polymer batteries are considered as one of the next-generation batteries by improving the weaknesses of lithium-ion batteries. , the discharge capacity at low temperatures is insufficient, and improvements are urgently required.

Such electrochemical devices are produced by many manufacturers, but their safety characteristics are different. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration is that the user should not be injured when the electrochemical device malfunctions, and for this purpose, the safety standards strictly regulate ignition and smoke generation in the battery. In terms of the safety characteristics of the electrochemical device, there is a high risk of explosion if the electrochemical device overheats and causes thermal runaway or if the separator is penetrated.

In order to solve this problem, it is necessary to evaluate the stability of the separator. expense has been spent.

Therefore, there is a demand for development of a method capable of quickly evaluating whether a separation membrane satisfies safety standards.

SUMMARY OF THE INVENTION An object of the present invention is to provide a safety evaluation method for a separation membrane that can predict explosion stability by simple rheology evaluation.

In order to achieve the above objects, according to one aspect of the present invention, a method for evaluating the stability of a separation membrane according to the following embodiments is provided.

A first embodiment is
preparing a separation membrane;
measuring the stretched physical properties of the separation membrane using dynamic mechanical analysis (DMA);
comparing the measured values with the stability criteria of the stretched physical properties;
As a result of the comparison, if the measured result value satisfies the stability criterion, the separation membrane is classified as a stability acceptable separation membrane, and if the stability criterion is not satisfied, the separation membrane is classified as a stability failure separation membrane. sorting into membranes;
The stretched physical properties are the breaking temperature and the shrinkage rate of the separation membrane, and the stability criteria are defined as a breaking temperature of 200° C. or more and a shrinkage rate of 59% or less, or
It relates to a method for evaluating the stability of a separation membrane, wherein the stretching properties are the breaking load and breaking elongation of the separation membrane, and the stability criteria are defined as a breaking load of 0.02 N or more and a breaking elongation of 1% or more. .

In the second embodiment, in the first embodiment,
The separation membrane that passed the stability test and the separation membrane that failed the stability test were subjected to pass and fail evaluations, respectively, when a nail penetration test was performed on a secondary battery equipped with the same separation membrane,
In the nail penetration test, if the secondary battery is fully charged at 25 ° C. with a voltage of 4.25 V and a nail with a diameter of 3 mm is used to pierce the center of the battery, it fails if it ignites, and if it does not ignite It relates to the stability evaluation method of the separation membrane described in the first embodiment, which is a method for evaluating the acceptance.

In the third embodiment, in the first embodiment or the second embodiment,
The separation membrane is located on the separation membrane of the porous polymer substrate and at least one surface of the porous polymer substrate, and is located on a plurality of inorganic particles and part or all of the surfaces of the inorganic particles. The present invention relates to a method for evaluating the stability of an organic/inorganic composite separation membrane having a porous coating layer containing a binder polymer that connects and fixes the inorganic particles, or a separation membrane including a mixed separation membrane thereof.

A fourth embodiment is any one of the first to third embodiments,
The method for measuring the breaking temperature of the separation membrane is to use dynamic mechanical analysis to measure a separation membrane sample with a width of 6.1 mm in a temperature range of 25 to 350 ° C. at a heating rate of 5 ° C./min to 0.005 N. The present invention relates to a method for evaluating the stability of a separation membrane by measuring the temperature at which a separation membrane sample breaks or stretches under load.

A fifth embodiment is any one of the first to fourth embodiments,
The shrinkage rate measurement method of the separation membrane uses dynamic mechanical analysis to measure a separation membrane sample with a width of 6.1 mm in a temperature range of 25 to 350 ° C. at a heating rate of 5 ° C./min and a load of 0.005 N. At this time, the shrinkage rate is calculated by [(initial length of separation membrane sample) - (minimum length of separation membrane sample) / (initial length of separation membrane sample) x 100]. The present invention relates to a method for evaluating the stability of a separation membrane.

A sixth embodiment is any one of the first to fifth embodiments,
The breaking load and breaking elongation rate of the separation membrane are measured using dynamic mechanical analysis. It relates to a method for evaluating the stability of a separation membrane, in which the load and elongation at the time of breakage are measured in increments of 0.001N.

According to an embodiment of the present invention, in order to select a separation membrane with excellent explosion safety, a significant factor of rheological characteristics affecting safety is derived, and the stability of the separation membrane can be easily and quickly determined. can be predicted.

Since the method for evaluating the stability of the separation membrane of the present invention provides a result that matches the nail penetration test of a secondary battery having the same separation membrane, it is not necessary to assemble the secondary battery separately and evaluate the stability. good.

The following drawings attached to this specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of the present invention together with the detailed description of the invention. It should not be construed as being limited only to the matters described in the drawings.

4 is a graph showing the deformation rate and static load during the breaking test of the separation membrane. 5 is a graph showing changes in deformation rate according to temperature of separation membranes of samples A to F; 4 is a graph showing shrinkage rates and rupture temperatures of separation membranes of samples A to L; 4 is a graph showing breaking loads and breaking elongations of separation membranes of samples A to L. FIG.

Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Prior to this, the terms and words used in the specification and claims should not be construed as being limited to their ordinary or dictionary meaning, and the inventors themselves have It should be interpreted with the meaning and concept according to the technical idea of the present invention according to the principle that the concept of the term can be properly defined.

Generally, a porous polymer substrate is applied to a separator used in a secondary battery. The material behaves like a liquid, and the separator applied to the battery is damaged in a nail penetration test, which greatly reduces the explosion safety of the secondary battery.

Therefore, among the rheological property values of the porous polymer substrate that constitutes the separation membrane of the present invention, the rheological property significant factor that is closely related to the prediction of the safety of the secondary battery is derived and evaluated. The purpose is to predict the stability of the final secondary battery.

A method for evaluating the stability of a separation membrane according to one aspect of the present invention includes:
preparing a separation membrane;
measuring the stretched physical properties of the separator using dynamic mechanical analysis (DMA) and comparing the measured values with the stability criteria of the stretched physical properties;
When the comparison result and the measured result value meet the stability criteria, the separation membrane is classified as a stability acceptable separation membrane, and when the stability criteria are not met, the stability rejection membrane is classified. and classifying into
The stretched physical properties are the breaking temperature and the shrinkage rate of the separation membrane, and the stability criteria are defined as a breaking temperature of 200° C. or more and a shrinkage rate of 59% or less, or
The elongation properties are the breaking load and breaking elongation of the separator, and the stability criteria are defined as breaking load of 0.02 N or more and breaking elongation of 1% or more.
First, a separation membrane to be evaluated for stability is prepared.

The separator includes a porous polymer substrate separator, an organic/inorganic composite separator having a porous coating layer located on at least one surface of a porous polymer substrate, or a mixed separator thereof. obtain.

Polymers constituting the porous polymer substrate can be applied without limitation as long as they exhibit the melting properties described above. Non-limiting examples of such polymers include polyolefins and modified polyolefins. etc. alone or in mixtures of two or more thereof. In addition, when two or more of the above polymers are used, they may be mixed in a single layer to form a porous polymer substrate, or a composite of two or more layers in which different polymers form separate layers. It may be a layer, and in this case, at least one of the composite layers may contain a mixture of two or more polymers.

At this time, the polyolefin may be polyethylene such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, or polyolefin-based polymers such as polypropylene, polybutylene, and polypentene, either alone or in combination of two or more thereof. can be mixed to form a polymer.

The modified polyolefin may be a copolymer of an olefin (eg, ethylene, propylene, etc.) and an α-olefin having 2 to 20 carbon atoms. The α-olefins include 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and It may be any one or more selected from the group consisting of 1-eicosene, or may have a structure containing one or more vinyl groups, ketone groups, ester groups, acid groups, etc. in the polymer chain. . In the ethylene/α-olefin copolymer, the α-olefin content may be about 0.5-10 wt%, preferably about 1-5 wt%, but is not limited thereto.

According to an embodiment of the present invention, the polyethylene includes ultra high molecular weight polyethylene; polyethylene other than high molecular weight polyethylene; could be. In this case, the ultra-high molecular weight polyethylene can be an ethylene homopolymer or a copolymer containing small amounts of α-olefins. The α-olefin may be in the form of having one or more branches of vinyl group, ketone group, methyl group, ester group, acidic group and the like in the polymer main chain.

The polyethylene other than the high-molecular-weight polyethylene may be any one of high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene.

Also, according to an embodiment of the present invention, the polypropylene may be a propylene homopolymer or a copolymer containing an α-olefin. The α-olefin is as previously described.

According to an embodiment of the present invention, the polymer may be a mixture of polyethylene and polypropylene, and polypropylene may be included within 5% by weight of the total polymer content. At this time, the polyethylene and polypropylene used are as described above.

In addition to polyolefin, the porous polymer substrate may be polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide. polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, polyethylenenaphthalene, or the like alone or as a mixture thereof can.

In order for the porous polymer substrate to have the above-described improved rheological properties, the polymer forming the porous polymer substrate has a predetermined Z-average molecular weight, melt index, and branch content. is required.

According to one embodiment of the present invention, the polymer has a Z-average molecular weight Mz of 500,000 to 2,000,000, particularly 600,000 to 1,800,000, more particularly 800,000 can be ~1,300,000.

The thickness of the porous polymer substrate is not particularly limited, but is specifically 1 to 100 μm, more specifically 5 to 50 μm, and the pore size and porosity of the porous polymer substrate are also not particularly limited. , respectively 0.01 to 50 μm and 10 to 95%.

According to an embodiment of the present invention, the organic/inorganic composite separation membrane is located on at least one surface of the porous polymer substrate and includes a plurality of inorganic particles and a part or surface of the inorganic particles. It may further include a porous coating layer containing a binder polymer positioned over the entire surface to connect and fix the inorganic particles.

As the binder polymer used for forming the porous coating layer, polymers commonly used for forming the porous coating layer in the art may be used. In particular, a polymer with a glass transition temperature (Tg) of −200 to 200° C. may be used, which may affect mechanical properties such as flexibility and elasticity of the finally formed porous coating layer. This is because the physical properties can be improved. The binder polymer serves as a binder that connects and stably fixes the inorganic particles to each other, thereby contributing to prevention of deterioration of the mechanical properties of the separator having the porous coating layer.

In addition, the binder polymer does not necessarily have to have ion-conducting ability, but when a polymer having ion-conducting ability is used, the performance of the electrochemical device can be further improved. Therefore, it is desirable to use a binder polymer having a dielectric constant as high as possible. In fact, since the dissociation degree of the salt in the electrolyte depends on the dielectric constant of the electrolyte solvent, the higher the dielectric constant of the binder polymer, the more the dissociation degree of the salt in the electrolyte can be improved. The dielectric constant of such a binder polymer can be in the range of 1.0 to 100 (measurement frequency=1 kHz), especially 10 or more.

In addition to the functions described above, the binder polymer has a feature of exhibiting a high degree of swelling by gelling when impregnated with a liquid electrolyte. Accordingly, the solubility index, ie Hildebrand solubility parameter, of the binder polymer is in the range of 15-45 MPa ^1/2 or 15-25 MPa ^1/2 and 30-45 MPa ^1/2 . Therefore, hydrophilic polymers having more polar groups than hydrophobic polymers such as polyolefins can be used. This is because when the solubility index is less than 15 MPa ^1/2 or more than 45 MPa ^1/2 , it may be difficult to swell with a typical liquid electrolyte for batteries.

Non-limiting examples of such binder polymers include polyvinylidene fluoride-co-hexafluoropropylene, vinylidene fluoride-co-trichloroethylene. , polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-vinyl acetate copolymer-polyethylene oxide, （ｐｏｌｙｅｔｈｙｌｅｎｅ ｏｘｉｄｅ）、ポリアリレート（ｐｏｌｙａｒｙｌａｔｅ）、セルロースアセテート（ｃｅｌｌｕｌｏｓｅ ａｃｅｔａｔｅ）、セルロースアセテートブチレート（ｃｅｌｌｕｌｏｓｅ ａｃｅｔａｔｅ ｂｕｔｙｒａｔｅ）セルロースアセテートプロピオネート（ｃｅｌｌｕｌｏｓｅ ａｃｅｔａｔｅ ｐｒｏｐｉｏｎａｔｅ）、シアノエチルプルラン（ｃｙａｎｏｅｔｈｙｌｐｕｌｌｕｌａｎ）、シアノエチルポリビニルアルコール（ｃｙａｎｏｅｔｈｙｌｐｏｌｙｖｉｎｙｌａｌｃｏｈｏｌ） , cyanoethylcellulose, cyanoethylsucrose, pullulan and carboxyl methyl cellulose, and the like.

The weight ratio of the inorganic particles to the binder polymer is, for example, 0:50 to 99:1, specifically 70:30 to 95:5. When the content ratio of the inorganic particles to the binder polymer satisfies the above range, it is possible to prevent the problem that the pore size and porosity of the formed coating layer are reduced due to the increase in the content of the binder polymer. Since the content of molecules is small, the problem of weak peeling resistance of the formed coating layer can be solved.

The separator according to one aspect of the present invention may further include other additives in addition to the inorganic particles and polymers described above as components of the porous coating layer.

The inorganic particles of the present invention include high dielectric constant inorganic particles having a dielectric constant of 5 or more, specifically 10 or more, inorganic particles having lithium ion transfer ability, or mixtures thereof.

Non-limiting examples of inorganic particles having a dielectric constant of 5 or more include BaTiO ₃ , Pb(Zr, Ti)O ₃ (PZT), Pb _1-x La _x Zr _1-y Ti _y O ₃ ( PLZT), _PB (Mg1 _{/3Nb2 /3} )O3 _{- PbTiO3} (PMN-PT), hafnia ( _HfO2 ), _SrTiO3 , SnO2, _CeO2 , MgO, _NiO , CaO, ZnO, ZrO _{2 , Y2O3} , _Al2O3 , AlO(OH) _{, Al2O3.H2O} , _{TiO2 , SiC} , or mixtures thereof.

As used herein, the term "inorganic particles having lithium ion transfer ability" refers to inorganic particles that contain lithium elements but do not store lithium and have the function of transferring lithium ions, and have lithium ion transfer ability. Non-limiting examples of inorganic particles include lithium phosphate ( _Li3PO4 ), lithium titanium phosphate ( _LixTiy ( _PO4 ) _{3 ,} 0<x< ₂ , 0<y<3), lithium aluminum titanium Phosphate ( _LixAlyTiz ( _{PO4 ) 3} , 0<x<2 _{, 0} <y< ₁ , ₀ <z<3), _14Li2O - _{9Al2O3-38TiO2-39P2O5} , _etc. (LiAlTiP) _{x Oy} -based glass (0<x<4, 0<y<13 ₎ , lithium lanthanum titanate ( _LixLayTiO3 , 0<x< ₂ , 0<y<3), Li _3. Lithium germanium thiophosphates ( _LixGeyPzSw , 0 _{< x <4} , 0<y<1, 0<z<1, 0 such as _{25Ge0.25P0.75S4 ,} etc. ₎ <w<5), Lithium Nitride (Li _x N _y , 0<x<4, 0<y<2) such as Li ₃ N, Li ₃ PO ₄ —Li ₂ S—SiS _{2 ,} etc. _SiS2- based glass (Li _x Si _y S _z , 0<x<3 _, 0<y<2, 0 _< z<4 ₎ , _P2S5 -based such as LiI- _Li2SP2S5 glass (Li _x P _y S _z , 0<x<3, 0<y<3, 0<z<7) or mixtures thereof;

According to one embodiment of the present invention, the average particle size of the inorganic particles may be 0.05-3 μm, specifically 0.1-2.7 μm, more specifically 0.5-2.5 μm.

The thickness of the porous coating layer is not particularly limited, but specifically 1 to 10 μm, more specifically 1.5 to 6 μm, and the porosity of the porous coating layer is also not particularly limited, but is 35 to 65%. It is desirable to have

Next, the stretching properties of the separation membrane are measured using dynamic mechanical analysis (DMA).

The stretching property of the separator can be measured by a standard method using dynamic mechanical analysis equipment, specifically, Q800 DMA equipment of TA Instruments (USA).
The stretched physical properties are the rupture temperature and shrinkage of the separation membrane, and the stability criteria are defined as a rupture temperature of 200° C. or higher and a shrinkage of 59% or less, or the stretched physical properties are defined as the separation membrane. and the stability criterion is defined as a breaking load of 0.02 N or more and a breaking elongation of 1% or more.

First, the case where the stretched physical properties are defined as the breaking temperature and shrinkage ratio of the separator, and the stability criterion is defined as the breaking temperature of 200° C. or higher and the shrinkage ratio of 59% or lower will be described.

According to an embodiment of the present invention, the breaking temperature of the separation membrane is a tension mode of the DMA equipment, a separation membrane sample having a width of 6.1 mm is prepared, and the heating rate is 5. C./min, the temperature range is 25-350.degree. C., and the applied load is 0.005N.

The length of the specimen can be adjusted appropriately according to the test equipment, and can be, for example, 8-20 mm, specifically 10-11 mm, more specifically 10.3 mm. In addition, the thickness of the test piece can be appropriately adjusted according to the test equipment, and can be, for example, 20 μm or less, specifically 7-20 μm.

In addition, the above-mentioned "temperature at which the separation membrane sample stretches" means that the deformation rate of the separation membrane sample within a temperature change of 5 ° C. is in the + direction in the deformation rate graph (see FIG. 2) due to temperature change in dynamic mechanical analysis (DMA). Defined as the temperature at which it increases by 10% or more.

The shrinkage rate of the separation membrane is measured by dynamic mechanical analysis using a 6.1 mm wide separation membrane sample at a temperature range of 25 to 350° C. at a heating rate of 5° C./min and a load of 0.005 N. The shrinkage rate at this time can be calculated by [(initial length of separation membrane sample)−(minimum length of separation membrane sample)/(initial length of separation membrane sample)×100]. The above-mentioned "minimum length of the separation membrane sample" refers to the "length of the separation membrane sample" when the separation membrane sample shrinks due to heat and has the shortest length when the temperature of the separation membrane sample is raised under the above conditions using dynamic mechanical analysis. means. The minimum length can be the length just before the separation membrane sample breaks or expands in length if the separation membrane stability evaluation receives a stability failure decision. The measured result values are compared with stability criteria defined as a breaking temperature of 200° C. or higher and a shrinkage of 59% or lower.

As a result of the measurement, the fact that the separation membrane satisfies both the breaking temperature of 200° C. or higher and the shrinkage rate of 59% or less means that the separation membrane is not easily deformed by heat generated when the positive electrode and the negative electrode face each other. Since the contraction rate of the separation membrane is small, the gaps and spaces that can be generated inside the battery due to the volume change of the separation membrane are small, so it is possible to prevent the generation of greater heat and explosion, resulting in the stability of the separation membrane. and that the stability of the battery is ensured.

As a result of comparing the measured result value with the stability criteria defined as a breaking temperature of 200 ° C. or more and a shrinkage rate of 59% or less, if the measured result value satisfies the stability criteria, it is stable If the above stability criteria are not met, the separation membrane is classified as a failing stability membrane.

Next, the case where the stretching properties are the breaking load and breaking elongation of the separation membrane and the stability criterion is defined as a breaking load of 0.02 N or more and a breaking elongation of 1% or more will be described.

According to one embodiment of the present invention, the breaking load and breaking elongation rate of the separation membrane are measured using dynamic mechanical analysis, and a separation membrane sample having a width of 6.1 mm and a length of 10 mm is heated at 200°C. The load and elongation at breakage can be measured while increasing the temperature from 0.002N by 0.001N per minute.

FIG. 1 is a graph showing the deformation rate and static load during a separation membrane breaking test. Referring to FIG. 1, the load and elongation at the time of occurrence of breakage means that the separation membrane breaks without further deformation in the process of increasing the load applied to deform the length of the separation membrane of a predetermined standard. means the load and deformation rate at the time of

The length of the specimen can be adjusted appropriately according to the test equipment, and can be, for example, 8-20 mm, specifically 10-11 mm, more specifically 10.3 mm. Also, the thickness of the test piece can be appropriately adjusted according to the test equipment, and can be, for example, 20 μm or less, specifically 7 to 20 μm.

The measured result values are compared with a stability criterion defined as a breaking load of 0.02 N or more and a breaking elongation of 1% or more.

As a result of the measurement, the separator has a breaking load of 0.02N or more and a breaking elongation of 1% or more, which means that the battery is less likely to be damaged by external impact or heat, and the explosion stability of the battery is increased. do.

As a result of comparing the measured result value with a stability criterion defined as a breaking load of 0.02 N or more and a breaking elongation rate of 1% or more, if the measured result value satisfies the stability criterion , the separation membrane is classified as a stability-passed separation membrane, and if it does not meet the stability criteria, it is classified as a stability-failed separation membrane.

The separation membranes that passed the stability test and the separation membranes that failed the stability test were evaluated as pass and fail, respectively, when a secondary battery equipped with the same separation membrane was subjected to a nail penetration test.

During the nail penetration test, the positive electrode and the negative electrode face each other and an exothermic reaction occurs, and the exothermic temperature at this time is quite high. Therefore, in the method for evaluating the stability of the separation membrane of the present invention, the thermal environment generated during the actual nail penetration test is reflected to the greatest extent possible, and from such evaluation results, breakage occurs at high temperatures. It is expected that a battery using a separator that is determined to have a small shrinkage rate without shrinkage will receive the same stability evaluation during the nail penetration test.

As a result, the method for evaluating the stability of the separation membrane of the present invention is consistent with the results of the nail penetration test of the secondary battery having the separation membrane. The safety of the secondary battery can be predicted without performing a test by assembling.

At this time, in the nail penetration test, the secondary battery was fully charged at 25° C. and a voltage of 4.25 V, and a nail with a diameter of 3 mm was used to pierce the center of the battery. , If it does not ignite, it is evaluated as a pass.

The above-mentioned secondary battery is manufactured by constructing an electrode assembly by using a normal cathode and an anode as two electrodes, sandwiching a separation membrane between them, housing this in a battery case, and injecting an electrolyte solution. can be done.

At this time, the applicable cathode and anode are not particularly limited, and the electrode active material may be bound to the electrode current collector by a conventional method known in the art. Non-limiting examples of the cathode active material among the electrode active materials include common cathode active materials used in cathodes of conventional electrochemical devices, particularly lithium manganese oxide and lithium cobalt oxide. It is desirable to use a material, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide in which these are combined. Non-limiting examples of anode active materials can be conventional anode active materials used in anodes of conventional electrochemical devices, in particular lithium metal or lithium alloys, carbon, petroleum coke. , activated carbon, graphite or other carbons, and the like. Non-limiting examples of cathodic current collectors include foils made from aluminum, nickel, or combinations thereof; non-limiting examples of anodic current collectors include copper, gold, nickel, or Examples include foils made from copper alloys or combinations thereof.

The electrolyte usable in the secondary battery is a salt having a structure such as A ^{+ B-} , where A ⁺ is an alkali metal cation such as Li ⁺ , Na ⁺ , K ⁺ or a combination thereof. ^B- includes ions such as PF ₆ ^- , BF ₄ ^- , Cl ^- , Br ^- , I ^- , ClO ₄ ^- , AsF ₆ ^- , CH ₃ CO ₂ ^- , CF ₃ SO ₃ ^- , N(CF ₃ Salts containing anions such as SO ₂ ) ₂ ⁻ , C(CF ₂ SO ₂ ) ₃ ⁻ or combinations thereof, such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), Dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), γ-butyrolactone or these Dissolved or dissociated in an organic solvent consisting of a mixture of, but not limited to.

The injection of the electrolyte may be performed at an appropriate stage in the manufacturing process of the battery, depending on the manufacturing process and required physical properties of the final product. That is, it can be applied before assembling the battery or at the final stage of assembling the battery.

Hereinafter, the present invention will be described with specific examples. However, embodiments according to the present invention can be modified in many other forms, and should not be construed as limiting the scope of the present invention to the embodiments set forth below. Rather, the embodiments of the present invention are provided so that this disclosure will be more thorough and complete for those of ordinary skill in the art.

Sample A
As a separation membrane for sample A, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.

<Production of cathode and anode>
96.7 parts by weight of Li[Ni _0.6 Mn _0.2 Co _0.2 ]O ₂ functioning as a cathode active material, 1.3 parts by weight of graphite functioning as a conductive material, polyvinylidene functioning as a binder 2.0 parts by weight of fluoride (PVdF) was mixed to prepare a cathode mixture. A cathode mixture slurry was prepared by dispersing the obtained cathode mixture in 1-methyl-2-pyrrolidone functioning as a solvent. The slurry was coated on both sides of an aluminum foil having a thickness of 20 μm, dried and pressed to prepare a cathode.

97.6 parts by weight of artificial graphite and natural graphite (weight ratio 90:10) functioning as an anode active material, 1.2 parts by weight of styrene-butadiene rubber (SBR) functioning as a binder, and carboxymethyl cellulose (CMC). 1.2 parts by weight were mixed to prepare an anode mixture. An anode mixture slurry was prepared by dispersing this anode mixture in ion-exchanged water functioning as a solvent. This slurry was coated on both sides of a copper foil having a thickness of 20 μm, dried and pressed to produce an anode.

<Production of lithium secondary battery>
LiPF ₆ was dissolved to a concentration of 1.0 M in an organic solvent in which ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) were mixed at a composition of 3:3:4 (volume ratio). A non-aqueous electrolyte was prepared.

The prepared cathode and anode were laminated so that the separation membrane of Sample A was interposed between them, which were placed in a pouch, and then the electrolyte was injected to prepare a lithium secondary battery of Sample A. .

sample B
As a separation membrane for sample B, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample B was manufactured in the same manner as sample A except that the separation membrane of sample B was used as the separation membrane.

Sample C
As a separation membrane for sample C, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample C was manufactured in the same manner as sample A except that the separation membrane of sample C was used as the separation membrane.

sample D
As a separation membrane for sample D, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample D was manufactured in the same manner as sample A except that the separation membrane of sample D was used as the separation membrane.

sample E
As a separation membrane for sample E, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample E was manufactured in the same manner as sample A except that the separation membrane of sample E was used as the separation membrane.

Sample F.
A polyethylene porous membrane having an organic/inorganic porous coating layer was prepared as the separation membrane of sample F, and the separation membrane of sample A was used as the polyethylene porous membrane.
A secondary battery of sample F was manufactured in the same manner as sample A except that the separation membrane of sample F was used as the separation membrane.

Specifically, the separation membrane of sample F was manufactured as follows.

Add 16 parts by weight of PVdF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer) as a binder polymer to 184 parts by weight of acetone at a solid content of 8% by weight, and dissolve at 50° C. for about 12 hours or more. to prepare a binder polymer solution. Alumina was added to the prepared binder polymer solution so that the weight ratio between the binder polymer and alumina (Al ₂ O ₃ ) having an average particle size of 500 nm as inorganic particles was 10:90, and dispersed to form porous particles. A slurry for a separation membrane was produced.

The prepared slurry was coated on both sides of the separation membrane of Sample A by a dip coating method, and the coating thickness was adjusted to about 4 μm to prepare a separator having porous coating layers on both sides. did.

sample G
As a separation membrane for sample G, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample G was manufactured in the same manner as sample A except that the separation membrane of sample G was used as the separation membrane.

Sample H
A polyethylene porous membrane having an organic/inorganic porous coating layer was prepared as the separation membrane of sample H, and the separation membrane of sample C was used as the polyethylene porous membrane.
A secondary battery of sample H was manufactured in the same manner as sample A except that the separation membrane of sample H was used as the separation membrane.

Specifically, the separation membrane of sample H was manufactured as follows.

16 parts by weight of PVdF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer) as a binder polymer is added to 184 parts by weight of acetone at a solid content of 8% by weight, and dissolved at 50° C. for about 12 hours or more. Thus, a binder polymer solution was prepared. Alumina was added to the prepared binder polymer solution so that the weight ratio between the binder polymer and alumina (Al ₂ O ₃ ) having an average particle size of 500 nm as inorganic particles was 10:90, and dispersed to form porous particles. A slurry for a separation membrane was produced.

The prepared slurry was coated on both sides of the separation membrane of Sample C by a dip coating method to adjust the coating thickness to about 4 μm, thereby manufacturing a separator having porous coating layers on both sides.

Sample I
As a separation membrane for sample I, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample I was manufactured in the same manner as sample A, except that the separation membrane of sample I was used as the separation membrane.

Sample J
As a separation membrane for sample J, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample J was manufactured in the same manner as sample A except that the separation membrane of sample J was used as the separation membrane.

Sample K
As a separation membrane for sample K, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample K was manufactured in the same manner as sample A except that the separation membrane of sample K was used as the separation membrane.

Sample L
As a separation membrane for sample L, a polyethylene porous membrane having physical properties shown in Table 1 below was prepared.
A secondary battery of sample K was manufactured in the same manner as sample A except that the separation membrane of sample L was used as the separation membrane.

In Table 1, the methods of measuring air permeability, porosity, weight and ER are as follows.

"Air permeability" means the time for 100 cc of air to permeate the separator, and the unit is second / 100 cc, and can be used interchangeably with permeation time , and is usually represented by a Gurley value or the like.

The air permeability was measured by the ASTM D726-94 method. The Gurley used here is the resistance to air flow and is measured by a Gurley densometer. The Gurley air permeability values described herein are given in seconds for 100 cc of air to pass through a 1 in ² cross-section under a pressure of 12.2 in H ₂ O, ie, vent time.

Porosity “Porosity” means the ratio of the volume occupied by pores to the volume of the separation membrane, and the porosity was measured according to ASTM D-2873.

Weight The separation membrane was cut to a standard of 1 m wide by 1 m long, and the weight was measured.

Separation membrane resistance (ER)
The separation membrane was sufficiently wetted with an electrolytic solution containing 1M LiPF ₆ at a volume ratio of EC (ethylene carbonate)/EMC (ethyl methyl carbonate)=1:2, and a coin cell was manufactured using only the separation membrane.

The manufactured coin cell was left at room temperature for 1 day, and the separation membrane resistance (ER) was measured by an impedance measurement method.

Evaluation Results (1) Evaluation of Breaking Temperature The breaking temperatures of the separation membranes of samples A to L were evaluated, and the results are shown in Table 2. Also, FIG. 2 shows a graph of the deformation rates of the separation membranes of samples A to F with increasing temperature.
At this time, the method for evaluating the breaking temperature was as follows.
-Evaluation equipment: Q800 device manufactured by TA -Evaluation temperature range: 25 to 350 ° C
-Temperature increase rate: 5°C/min
- Applied load: 0.005N
- Separation membrane sample specification: Width 6.1 mm
- Initial length of separation membrane sample: 10.3 mm

(2) Shrinkage Rate Evaluation The shrinkage rates of the separation membranes of samples A to L were evaluated, and the results are shown in Table 2. FIG. 3 shows a graph of the rupture temperature against the contraction rate of the separation membranes of samples A to L.

At this time, the shrinkage rate evaluation method was as follows.

Using dynamic mechanical analysis (DMA), a separation membrane sample with a width of 6.1 mm was measured at a temperature range of 25 to 350 ° C. at a heating rate of 5 ° C./min and a load of 0.005 N to shrink the shrinkage rate. was measured.

At this time, the shrinkage rate was calculated by [(initial length of separation membrane sample)−(minimum length of separation membrane sample)/(initial length of separation membrane sample)×100].

(3) Evaluation of Breaking Load and Breaking Elongation Rate The breaking load and breaking elongation rate of the separation membranes of samples A to L were evaluated, and the results are shown in Table 2. FIG. 4 shows a graph of the breaking load against the breaking elongation of the separation membranes of samples A to L.

At this time, the evaluation methods for the breaking load and breaking elongation rate were as follows.
- Evaluation equipment: DMA tensile geometry (manufacturer: TA, product name: Q800)
-Evaluation temperature: maintained at 200°C -Separation membrane sample specification: Width 6.1 mm (fixed),
- Initial length of separation membrane sample: 10 mm
- The separation membrane sample was loaded into the DMA tensile geometry, and the load was increased from 0.002 N to 0.001 N per minute to measure the load and elongation until breakage occurred. - The elongation at the breaking point of the separation membrane sample. and record the load and compare physical properties

(4) Stability evaluation by nail penetration test of secondary battery The secondary batteries of samples A to L were fully charged at 4.25 V at 25° C., and a nail with a diameter of 3 mm was used to pierce the center of the battery. After that, the presence or absence of ignition was observed. At this time, the penetration speed of the nail was set to 80 mm/sec. The results are shown in Table 2 below.

Referring to Table 2, in the evaluation results of the breaking temperature and shrinkage rate of the separation membrane using DMA, if the breaking temperature is 200° C. or more and the shrinkage rate is 59% or less, the separation membrane is provided. The secondary battery also showed acceptable results in the nail penetration test. If not more than one, the secondary battery with such a separator failed the nail penetration test.

In addition, in the evaluation results of the breaking load and breaking elongation rate of the separation membrane using DMA, when both the conditions of a breaking load of 0.02 N or more and a breaking elongation rate of 1% or more are satisfied, such a separation membrane is provided. The secondary battery also showed acceptable results in the nail penetration test. , the secondary battery having such a separator failed the nail penetration test.

Although the samples F, H, and D did not break when the breaking temperature was measured, the fact that the breaking temperature was shown as 350°C in FIG. The temperature range was 25 to 350°C at that time, and it did not break even at the maximum temperature of 350°C, so it was expressed as 350°C.

Moreover, FIG. 4 shows the evaluation results of the breaking load and the breaking elongation. Referring to FIG. 4, samples B, J, D, H, and F passed the nail penetration test. B, J, D and H were fractured. Referring to FIG. 4 for broken samples B, J, D, and H, in terms of breaking load and breaking elongation, samples H>D>J>B show high breaking load and elongation in the order, Relative stability can be compared in more detail.

From this, according to the method for evaluating the stability of a separation membrane according to the present invention, the evaluation results of the DMA breaking temperature and shrinkage rate of the separation membrane itself can be obtained without actually assembling a secondary battery and performing a nail penetration test. Alternatively, it can be seen that it can be accurately predicted from the evaluation results of the breaking load and breaking elongation.

Claims (3)

preparing a separation membrane;
measuring the stretched physical properties of the separation membrane using dynamic mechanical analysis (DMA);
comparing the measured values with the stability criteria of the stretched physical properties;
As a result of the comparison, if the measured result value satisfies the stability criteria, it is predicted that the separation membrane passes the stability test in the nail penetration test of the secondary battery equipped with the same separation membrane, Predicting that if the stability criteria are not met, the separation membrane fails stability in a nail penetration test of a secondary battery equipped with the same separation membrane ;
The stretching property is defined as the breaking load and breaking elongation rate of the separation membrane, and the stability criterion is defined as a breaking load of 0.02 N or more and a breaking elongation rate of 1% or more ,
Using dynamic mechanical analysis, the breaking load and breaking elongation rate of the separation membrane were 0.002 N to 0.001 N per minute at a temperature of 200 ° C. for a separation membrane sample with a width of 6.1 mm and a length of 10 mm. A method for evaluating the stability of a separation membrane obtained by a method of measuring the load and elongation rate at the time of breakage while increasing the load by increments. The separation membrane that passed the stability test and the separation membrane that failed the stability test were subjected to pass and fail evaluations, respectively, when a nail penetration test was performed on a secondary battery equipped with the same separation membrane,
In the nail penetration test, if the secondary battery is fully charged at 25 ° C. with a voltage of 4.25 V and a nail with a diameter of 3 mm is used to pierce the center of the battery, it fails if it ignites, and if it does not ignite 2. The method for evaluating the stability of a separation membrane according to claim 1, which is a method of evaluating as acceptable. The separation membrane is located on the separation membrane of the porous polymer substrate and at least one surface of the porous polymer substrate, and is located on a plurality of inorganic particles and part or all of the surfaces of the inorganic particles. 3. The stability of the separation membrane according to claim 1 or 2, comprising an organic/inorganic composite separation membrane having a porous coating layer containing a binder polymer that connects and fixes the inorganic particles, or a mixed separation membrane thereof. Evaluation method.

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